The I2C bus system is an electronic bus for carrying commands and data between compatible devices connected to the bus. The system was developed and marketed by Philips Semiconductor Corporation and is described in detail in the I2 C Specification, revision 2.0, Philips Semiconductor Corporation 1998, which specification is hereby incorporated by reference in its entirety. In the I2C bus system, two wires, called a serial data (SDA) line and serial clock (SCL) line, carry information between the devices connected to the bus. Both the SDA and SCL lines are bi-directional lines, connected to a positive supply voltage via pull-up resistors as shown in FIG. 1 to form a “wired-AND” configuration. For example, in the bus configuration 100 illustrated in FIG. 1, the SDA line 108 and the SCL line 110 are connected to the VDD supply line 102 by pull-up resistors 104 and 106, respectively. Other buses, which use a similar protocol, include the SMBus, Access.bus and the InfiniBandSM management link. Collectively, this type of bus system will be termed a “wired-AND” bus system. The remainder of the discussion will focus on the I2C bus system with the understanding that the discussion applies to these other bus systems as well.
When the bus 101 is free, both the SDA line 108 and the SCL line 110 are pulled to a “HIGH” state by the resistors 104 and 106. The output stages of devices connected to the bus must have an open-drain or open-collector in order to form the wired-AND configuration. Two devices 112 and 114 are shown schematically in FIG. 1. Device 112 has a clock output stage which includes output transistor 116 which is connected across the SCL line 110 and ground 118. When a signal on the gate 117 of transistor 116 turns the transistor on, it pulls the SCL line 110 “LOW”. Clock signals on the SCL line 110 can be detected by means of buffer 120 whose output forms the “clock in” line 122.
Similarly, device 112 has a data output stage which includes output transistor 124 which is connected across the SDA line 108 and ground 126. When a signal on the gate 123 of transistor 124 turns the transistor on, it pulls the SDA line 108 “LOW”. Data signals on the SDA line 108 can be detected by means of buffer 128 whose output forms the “data in” line 130. Device 114 also has a clock output transistor 132 and clock input buffer 134 and a data output transistor 136 and data input buffer 138 for communication with the SDA and SCL lines, 108 and 110.
Devices on the bus communicate by periodically pulling the SDA and SCL lines 108 and 110 LOW producing data and clock pulses on the lines 108 and 110. In accordance with the I2C protocol, the data on the SDA line 108 must be stable when the clock line SCL 110 is HIGH. Thus, the HIGH or LOW state of the data line 108 can only change when the clock line 110 is LOW. Two unique situations arise, which situations are defined as START and STOP conditions. In particular, a HIGH to LOW transition on the SDA line 108 while the SCL line 110 is HIGH is defined as a START condition. A LOW to HIGH transition on the SDA line 108 while the SCL line 110 is HIGH is defined as a STOP condition.
Each device 112, 114 on the bus 101 has a unique address and can operate as either a data transmitter or a data receiver, depending on the function of the device. For example, an LCD driver is always a data receiver, whereas a memory can both receive and transmit data. In addition to being transmitters and receivers, devices can also be bus masters or slaves when performing data transfers. A bus master is the device that initiates a data transfer on the bus, generates the clock signals required for that transfer and terminates that data transfer. During this transfer, any other device to which data is sent, or from which data is received, is considered a slave. The bus master may transmit data to a slave or receive data from a slave. In both cases, the clock signals are generated by the bus master. Bus master and slave relationships are not permanent and depend on which device initiated the data transfer at a given time.
More than one bus master device can be connected to bus 101. Bus implementations with exactly one device capable of acting as a master are called single-master buses, while those with two or more devices capable of acting as bus masters are called multimaster buses. In a single-master bus system, the I2C protocol is very straightforward, with every transaction consisting of a START condition followed by one or more address and data phases, followed by a STOP condition. Thus, the START and STOP conditions frame all activity on the bus and hence define the duration during which the bus is busy.
A slave device responds to an address or data phase generated by the master with either an acknowledgement (ACK) or a negative-acknowledgement (NAK) response. An ACK response is represented as a LOW signal level on the SDA line 108 during the acknowledge bit time, which is defined as the ninth clock pulse of any address or data phase. A NAK response is represented as a HIGH signal level on the SDA line 108 during the acknowledge bit time. Since the I2C bus is a wired-AND configuration where the buses are always HIGH unless pulled LOW by a device, a NAK response is equivalent to no response from a slave device. A NAK response during an address phase may indicate that the slave device is busy and unable to accept I2C transactions at this time, that it is non-functional or simply missing.
While a simple single-master system using the I2C technology works well, the situation becomes much more complicated when much larger and more complex I2C subsystems involving multiple unique master devices and dozens of slave devices on a single bus are constructed. Perhaps the biggest challenge in the design of any large multi-master I2C subsystem is the difficulty of ensuring reliable operation in the presence of multiple master implementations designed by different vendors. Since most I2C devices are used in simple single-master bus implementations, many available I2C master devices which claim to support multi-master operation have not been tested and verified sufficiently to be used together reliably on a single I2C bus. Furthermore, the wired-AND nature of the I2C bus means that a failure of any one device can cause the entire bus to fail, leading to difficulties in isolating the cause and nature of the fault that caused the bus failure.
In addition, one of the fundamental signal integrity challenges in any large I2C system design is meeting the rise-time specification of one microsecond on the SDA and SCL signals. Because these are open-collector signals, usually with a simple pull-up resistor to the supply rail, the rise time is proportional to the total capacitance on the bus. Further, the strength of the pull-up resistor is limited by the maximum current that the output cells on I2C components can sink, which is stated in the aforementioned I2C specification as 3 ma. Thus, the total capacitance that an I2C bus segment can tolerate is approximately 400 picofarads, beyond which the rise-time specification cannot be met. These design constraints limit both the number of master and slave devices on the bus as well as the physical length of the bus, making large I2C system designs very challenging.
Therefore there is need for a method and apparatus to reliably construct complicated I2C systems and to isolate faults so that they can be eliminated.